The present invention relates to a synchronization system for a key telephone system, in which a voice signal and a control signal having the form of a frequency shift signal are multiplexed in an intercom line for transmission.
In a conventional synchronization system for this kind of key telephone system, a no-carrier period is provided before a control signal from a key service unit to a key telephone set so that a start bit following the abovesaid period is detected, thereby to synchronize between transmission timings of the key service unit and the key telephone set. With this known system, however, since a receiver circuit is extremely susceptible to the influence of moise during the no-carrier period, there is the possibility that the synchronization is affected by noise, causing the key telephone set to malfunction thereafter. Since the no-carrier period also occurs in the case of changing the number of key telephone sets installed, there is also the liklihood that the head of a control signal from another key telephone set is recognized as the start bit of the key service unit. Further, in a receiving circuit, such as a phase lock loop (PLL), in which the lock-in time is far longer than the response time of reception, there is also the possibility that the start bit cannot be detected, resulting in an out of synchronization situation.